1. Field of the Invention
This invention relates to a method and apparatus for detection by means of ultrasonic waves of edge position, thickness or splice position of a material web moving between a transmitter and a receiver of ultrasonic waves.
2. Description of the Prior Art
Various techniques have been utilized to sense the edge position of a material web, including optical methods in which the edge region of the web is irradiated and the amount of interruption of a beam of light by the web is detected, and pneumatic methods for transparent or photo-sensitive webs in which a stream of air is directed to the web edge from underneath and the change in air pressure above the web edge is detected. In the Japanese Patent Application Provisional Publication No. 62-501520 (family of U.S. Pat. No. 4,901,292) another method is disclosed which senses the edge position by detecting a change in the incident signal to the receiver caused by insertion of the web edge between the transmitter and the receiver of ultrasonic waves that are placed facing one another. In this method the transmitter intermittently emits a series of pulses of ultrasonic wave which are received by the receiver directly or after reflection at the web or the receiver, and the direct waves are extracted in the limited sampling range for the peak value to be transformed into electric signal.
The optical methods cannot be applied to edge detection of transparent webs. On the other hand, in the pneumatic methods the web tends to accumulate the dirt due to air purity and suction of the ambient dust. Moreover, the output signal fluctuates because of undulation or winding in the moving web by change in air stream pressure, and detection becomes unstable due to slow response. In the method of the JPAPP No. 62-501520 sampling of the electrical signals transformed with the receiver is performed in a limited range excluding the reflected waves, so that sampling and reset of received signals in the receiver must be synchronous to the transmission time of the transmitter. Hence, variation in the distance between the transmitter and the receiver needs renewal of the timing signal and a circuit also is required for generation of the sampling and reset signals.
An effect of reflected waves gives rise to the standing wave, so-called beat phenomenon, due to interference with the direct wave. The standing wave cannot proceed because it results from superposition of two waves with an equal amplitude and the reverse directions of proceeding. Occurrence of the standing wave leads to variation in the received signals, making detection impossible. The interference of the direct waves with the reflected waves, however, causes no hindrance in detection of edge position, thickness and splice position of the web if the standing wave is kept from occurring. For their detection is possible by detection of the maximum value of the received waves, in which the direct waves with high peak values precede the reflected waves with lower peak values.